When designing PCB, we usually rely on the experience and skills we usually find on the Internet. Each PCB design can be optimized for a specific application. Generally, its design rules are only applicable to the target application. For example, the ADC PCB rules do not apply to RF PCBs and vice versa. However, some guidelines can be considered general for any PCB design. Here, in this tutorial, we will introduce some basic problems and skills that can significantly improve PCB design.
Power distribution is a key element in any electrical design. All your components rely on power to perform their functions. Depending on your design, some components may have different power connections, while some components on the same board may have poor power connections. For example, if all components are powered by one wiring, each component will observe a different impedance, resulting in multiple grounding references. For example, if you have two ADC circuits, one at the beginning and the other at the end, and both ADCs read an external voltage, each analog circuit will read a different potential relative to themselves.
We can summarize the power distribution in three possible ways: single point source, Star source and multipoint source.
(a) Single point power supply: the power supply and ground wire of each component are separated from each other. The power routing of all components only meets at a single reference point. A single point is considered suitable for power. However, this is not feasible for complex or large / medium-sized projects.
(b) Star source: Star source can be regarded as an improvement of single point source. Because of its key characteristics, it is different: the routing length between components is the same. Star connection is usually used for complex high-speed signal boards with various clocks. In the high-speed signal PCB, the signal usually comes from the edge and then reaches the center. All signals can be transmitted from the center to any area of the circuit board, and the delay between areas can be reduced.
(c) Multipoint sources: considered poor in any case. However, it is easy to use in any circuit. Multipoint sources may produce reference differences between components and in common impedance coupling. This design style also allows high switching IC, clock and RF circuits to introduce noise in nearby circuits sharing connections.
Of course, in our daily life, we will not always have a single type of distribution. The tradeoff we can make is to mix single point sources with multi-point sources. You can put analog sensitive devices and high-speed / RF systems in one point, and all other less sensitive peripherals in one point.
Have you ever thought about whether you should use power aircraft? The answer is yes. Power board is one of the methods to transfer power and reduce the noise of any circuit. The power plane shortens the grounding path, reduces the inductance and improves the electromagnetic compatibility (EMC) performance. It is also due to the fact that a parallel plate decoupling capacitor is also generated in the power supply planes on both sides, so as to prevent noise propagation.
The power board also has an obvious advantage: due to its large area, it allows more current to pass through, thus increasing the operating temperature range of the PCB. But please note: the power layer can improve the working temperature, but the wiring must also be considered. The tracking rules are given by ipc-2221 and ipc-9592
For a PCB with an RF source (or any high-speed signal application), you must have a complete ground plane to improve the performance of the circuit board. The signals must be located on different planes, and it is almost impossible to meet both requirements at the same time using two layers of plates. If you want to design an antenna or any low complexity RF board, you can use two layers. The following figure shows an illustration of how your PCB can better use these planes.
In mixed signal design, manufacturers usually recommend that analog ground be separated from digital ground. Sensitive analog circuits are easily affected by high-speed switches and signals. If analog and digital grounding are different, the grounding plane will be separated. However, it has the following disadvantages. We should pay attention to the crosstalk and loop area of the divided ground caused mainly by the discontinuity of the ground plane. The following illustration shows an example of two separate ground planes. On the left side, the return current cannot pass directly along the signal route, so there will be a loop area instead of being designed in the right loop area.
Electromagnetic compatibility and electromagnetic interference (EMI)
For high frequency designs (such as RF systems), EMI can be a major disadvantage. The ground plane discussed earlier helps reduce EMI, but according to your PCB, the ground plane may cause other problems. In laminates with four or more layers, the distance of the aircraft is very important. When the capacitance between planes is small, the electric field will expand on the board. At the same time, the impedance between the two planes decreases, allowing the return current to flow to the signal plane. This will produce EMI for any high frequency signal passing through the plane.
A simple solution to avoid EMI is to prevent high-speed signals from crossing multiple layers. Add decoupling capacitor; And place grounding vias around the signal wiring. The following figure shows a good PCB design with high frequency signal.
Bypass capacitors and ferrite beads are capacitors used to filter the noise generated by any component. Basically, if used in any high-speed application, any I / O pin may become a noise source. In order to make better use of these contents, we will have to pay attention to the following points:
Always place ferrite beads and bypass capacitors as close as possible to the noise source.
When we use automatic placement and automatic routing, we should consider the distance to check.
Avoid vias and any other routing between filters and components.
If there is a ground plane, use multiple through holes to ground it correctly.